Pressure-sensitive adhesive tapes often include release liners which are stripped off when the tapes are put to use. Widely used in release liners is biaxially-oriented poly(ethylene terephthalate) film base, being readily available, strong, smooth, dimensionally stable, and having good resistance to elevated temperatures. Preferred release materials include silicone coatings which adhere well to polyester film base immediately after being cured, but their adhesion becomes poor within a week or two. This poor adhesion is especially apparent when employed in release liners which are to be reused.
Solvent-free addition-cure silicones comprise a preferred class for release liners because (1) they avoid pollution or solvent-recovery problems and (2) they tend to release more easily from agressive pressure-sensitive adhesive layers than do other silicones. However, after a week or two they tend to adhere more poorly to polyester film base than do other silicones and hence are seldom used, in spite of the availability of a variety of known adhesion-promoting treatments for polyester film base. Such treatments include corona discharge, spark and other electrical discharges, flame, and physical and/or chemical etching. Other techniques for promoting adhesion to polyester film base employ organic priming or subbing coatings which can be better adhered to the film base by subjecting the coatings to ultraviolet radiation as taught in U.S. Pat. Nos. 3,188,266 (Charbonneau et al.) and 4,210,703 (Scantlin et al.).
U.S. Pat. No. 2,955,953 (Graham) concerns promoting adhesion to any solid organic polymeric substrate by subjecting it in the substantial absence of oxygen to charged particle ionizing radiation having an energy of from 15 to 50,000 electron volts. The accelerated particles may be utilized in a vacuum or may pass through a window and utilized in air or a gas. In the Example, cellophane was irradiated in a cathode ray tube, and only the face of the film exposed to the electron beam exhibited the desired improvement in adhesion. The Graham patent suggests that not more than five minutes should elapse before a coating is applied unless the irradiated substrate "is kept in an inert atmosphere such as under nitrogen, argon, helium or the like and/or is stored at a low temperature such as at -80.degree. C." (column 2, lines 1-6).
Japanese Patent Application JA55-160598 (Takada et al.) which was laid open May 26, 1982 concerns the application by vacuum deposition of a magnetizable thin film such as Co/Ni to polyester film. Immediately before applying the magnetizable thin film, the polyester film base is placed in a poly(ethylene terephthalate) tube or bag and subjected to electron-beam radiation having an energy of 2.5 Mev (Examples 1 and 2). Although none of the examples give other conditions of the preirradiation, the final paragraph of the application states that the preirradiation may be conducted in a vacuum container, followed immediately be deposition of the magnetizable thin film to keep the surface of the film base free from moisture and dust. By doing so, the Co/Ni or other magnetizable film is said to adhere better to the polyester film base. See also the claim and the penultimate paragraph immediately preceding Example 1.
In efforts to develop a silicone release liner having a polyester film base, it has been difficult to quantify the effectiveness of adhesion-promoting treatments in silicone-coated release liners. In one test an aggressively pressure-sensitive silicone adhesive tape is pressed against the silicone coating of the release liner, thus creating a bond between the tape and the silicone coating which is stronger than the bond between the silicone coating and the polyester film base. The force required to peel back the tape is recorded as a measure of how well the silicone coating is adhered to the polyester film base. Although peel adhesion is meaningful, it may not precisely indicate the resistance of a silicone coating to be removed when rubbed. A better indication of adequate adhesion to resist rubbing or abrasion failure has been obtained by vigorously rubbing the silicone coating with one's thumb. Any adhesion test is more meaningful if the silicone coating has been cured more than a week or two prior to the test.